Method for Making an Abrasion-Resistant Steel Plate and Plate Obtained

ABSTRACT

The invention concerns a method for making an abrasion resistant steel plate having a chemical composition comprising: 0.35%≦C≦0.8%, 0%≦Si≦2%, 0%≦Al≦2%, 0.35%≦Si+Al≦2%, 0%≦Mn≦2.5%, 0%≦Ni≦5%, 0%≦Cr≦5%, 0%≦Mo≦0.50%, 0%≦W≦1.00%, 0.1%≦Mo+W/2≦0.50%, 0%≦B≦0.02%, 0%≦Ti≦2%, 0%≦Zr≦4%, 0.05%≦Ti+Zr/2≦2%, 0%≦S≦0.15%, N≦0.03%; optionally from 0% to 1.5% of Cu; optionally Nb, Ta or V with Nb/2+Ta/4+V≦0.5%; optionally less than 0.1% of Se, Te, Ca, Bi or Pb; the rest being iron and impurities; the composition satisfying: 0.1%≦C*=C−Ti/4−Zr/8+7×N/8≦0.55% and 1.05×Mn+0.54×Ni+0.5O×Cr+0.3×(Mo+W/2) 1/2 +K&gt;1.8, with K=0.5 if B≧0.0005% and K=0 if B&lt;0.0005% and Ti+Zr/2−7×N/2≧0.05%; hardening after austenitization while cooling at a speed &gt;0.5° C/s between a temperature &gt;AC 3  and ranging between T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2) and T-50° C.; then at a core speed Vr&lt;115×ep −1.7  between T and 100° C., (ep=plate thickness in mm); cooling down to room temperature. The invention also concerns the resulting plate.

This is a divisional of application Ser. No. 10/535,418 filed May 19,2005, which is a §371 of PCT/FR03/03359 filed Nov. 13, 2003, which arehereby incorporated by reference.

The present invention relates to an abrasion-resistant steel and itsproduction method.

Abrasion-resistant steels are well known and are generally steels havinggreat hardness (of from 400 to 500 Brinell), having a martensiticstructure and containing from 0.12% to 0.3% of carbon. It is generallytaken that, in order to increase the wear-resistance, it is simplynecessary to increase the hardness, but that is done to the detriment ofother properties, such as, for example, suitability for welding orforming by bending. In order to obtain steels having both very goodwear-resistance and good suitability for use, therefore, means otherthan increasing the hardness have been sought.

Thus, it has been proposed in EP 0527 276 and U.S. Pat. No. 5,393,358 toimprove the abrasion-resistance of a steel which contains from 0.05% to0.45% of carbon, up to 1% of silicon, up to 2% of manganese, up to 2% ofcopper, up to 10% of nickel, up to 3% of chromium and up to 3% ofmolybdenum, boron, niobium and vanadium, by adding from 0.015% to 1.5%of titanium, in order to form coarse titanium carbides. That steel isquenched and consequently comprises a martensitic structure, theincrease in abrasion-resistance being obtained by the presence of coarsetitanium carbides. However, more particularly when the steel is cast inbars, that improvement is limited because, under the effect of abrasivestresses, the carbides become separated and no longer serve theirpurpose. Furthermore, in those steels, the presence of coarse titaniumcarbides inhibits ductility. Consequently, plates produced with thosesteels are difficult to planish and bend, which limits their possibleuses.

The object of the present invention is to overcome those disadvantagesby providing an abrasion-resistant steel plate which has good surfaceevenness and which, all things otherwise being equal, hasabrasion-resistance which is better than that of known steels.

To that end, the invention relates to a method for producing aworkpiece, and in particular a plate, of steel for abrasion, whosechemical composition comprises by weight:

0.35% ≤ C ≤ 0.8% 0% ≤ Si ≤ 2% 0% ≤ Al ≤ 2% 0.35% ≤ Si + Al ≤ 2%0% ≤ Mn ≤ 2.5% 0% ≤ Ni ≤ 5% 0% ≤ Cr ≤ 5% 0% ≤ Mo ≤ 0.50%0% ≤ W ≤ 1.00% 0.1% ≤ Mo + W/2 ≤ 0.50% 0% ≤ Cu ≤ 1.5%0% ≤ B ≤ 0.02% 0% ≤ Ti ≤ 2% 0% ≤ Zr ≤ 4% 0.05% ≤ Ti + Zr/2 ≤ 2%0% ≤ S ≤ 0.15% N ≤ 0.03%

-   -   optionally at least one element selected from Nb, Ta and V at        contents such that Nb/2+Ta/4+V≦0.5%,    -   optionally at least one element from Se, Te, Ca, Bi, Pb at        contents which are less than or equal to 0.1%,        the balance being iron and impurities resulting from the        production operation, the chemical composition further complying        with the following relationships, with C*=C−Ti/4−Zr/8+7×N/8:

0.10%≦C*≦0.55%

and:

Ti+Zr/2−7×N/2≧0.05%

and:

1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>1.8, or more advantageously2

with: K=0.5 if B≧0.0005% and K=0 if B<0.0005%;according to the method, the workpiece or the plate is subjected to athermal quenching processing operation which is carried out in the heatfor forming in the hot state, such as rolling or after austenitizationby reheating in a furnace, which consists in:

-   -   cooling the plate at a mean cooling rate greater than 0.5° C./s        between a temperature greater than AC₃ and a temperature of from        T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2) to T-50° C., the        temperature being expressed in ° C. and the contents of C*, Mn,        Ni, Cr, Mo and W being expressed as % by weight,    -   then cooling the plate at a mean core cooling rate        Vr<1150×ep^(−1.7) (in ° C./s) and greater than 0.1° C./s between        the temperature T and 100° C., ep being the thickness of the        plate expressed in mm,    -   and cooling the plate as far as ambient temperature, planishing        optionally being carried out.

Quenching may optionally be followed by tempering at a temperature ofless than 350° C., and preferably less than 250° C.

The invention also relates to a workpiece, and in particular a plate,obtained in particular by this method, the steel having a structurewhich is constituted by from 5% to 20% of retained austenite, theremainder of the structure being martensitic or martensitic/bainiticwith carbides. When the workpiece is a plate, its thickness may be from2 mm to 150 mm and its surface evenness may be characterized by adeflection which is less than or equal to 12 mm/m, and preferably lessthan 5 mm/m.

When the carbon content is such that:

0.1%≦C−Ti/4−Zr/8+7×N/8≦0.2%,

the hardness is preferably from 280 HB to 450 HB.

When the carbon content is such that:

0.2%<C−Ti/4−Zr/8+7×N/8≦0.3%,

the hardness is preferably from 380 HB to 550 HB.

When the carbon content is such that:

0.3%<C−Ti/4−Zr/8+7×N/8≦0.5%,

the hardness is preferably from 450 HB to 650 HB.

The invention will now be described in greater detail, but in anon-limiting manner, and illustrated with reference to examples.

In order to produce a plate according to the invention, a steel isproduced whose chemical composition comprises, in % by weight:

-   -   from 0.35% to 0.8% of carbon, and preferably more than 0.45%, or        more than 0.5%, and from 0% to 2% of titanium, from 0% to 4% of        zirconium, these contents having to be such that:        0.05%≦Ti+Zr/2≦2%. The carbon is intended, firstly, to achieve a        sufficiently hard martensitic structure and, secondly, to form        titanium and/or zirconium carbides. The total Ti+Zr/2 must be        greater than 0.05%, preferably greater than 0.10%, and, more        advantageously still, greater than 0.3%, or even greater than        0.5%, so that there is a minimum of carbides formed, but must        remain less than 2%, and preferably less than or equal to 0.9%,        because above that level the toughness and the suitability for        use are inhibited.    -   From 0% (or trace levels) to 2% of silicon and from 0% (or trace        levels) to 2% of aluminium, the total Si+Al being from 0.35% to        2% and preferably being greater than 0.5%, and more        advantageously still greater than 0.7%. Those elements, which        are deoxidants, further have the effect of promoting the        production of a metastable retained austenite which is heavily        charged with carbon and whose transformation into martensite is        accompanied by a large expansion promoting the anchoring of the        titanium carbides.    -   From 0% (or trace levels) to 2% or even 2.5% of manganese, from        0% (or trace levels) to 4% or even 5% of nickel and from 0% (or        trace levels) to 4% or even 5% of chromium, in order to obtain        an adequate level of quenchability and to adjust the various        mechanical characteristics or characteristics of use. Nickel has        in particular an advantageous effect on the strength, but that        element is expensive. Chromium also forms fine carbides in        martensite or bainite.    -   From 0% (or trace levels) to 0.50% of molybdenum. That element        increases the quenchability and forms, in martensite or bainite,        fine hardening carbides, in particular by precipitation owing to        auto-tempering during cooling. It is not necessary to exceed a        content of 0.50% in order to obtain the desired effect, in        particular with regard to the precipitation of hardening        carbides. Molybdenum can be replaced, completely or partially,        with twice the weight of tungsten. Nevertheless, this        substitution is not desirable in practice because it does not        provide any advantage over molybdenum and is more expensive.    -   Optionally, from 0% to 1.5% of copper. That element can bring        about an additional hardening without inhibiting the        weldability. Above a level of 1.5%, it no longer has any        significant effect, leads to hot-rolling difficulties and is        needlessly expensive.    -   From 0% to 0.02% of boron. That element can be added optionally        in order to increase the quenchability. So that that effect is        obtained, the content of boron must preferably be greater than        0.0005%, or more advantageously 0.001%, and does not need to        exceed substantially 0.01%.    -   Up to 0.15% of sulphur. That element is a residue which is        generally limited to 0.005% or less, but its content may be        voluntarily increased in order to improve machinability. It        should be noted that, in the presence of sulphur, in order to        prevent difficulties concerning transformation in the hot state,        the content of manganese must be greater than 7 times the        content of sulphur.    -   Optionally, at least one element selected from niobium, tantalum        and vanadium at contents such that Nb/2+Ta/4+V remains less than        0.5% in order to form relatively coarse carbides which improve        the resistance to abrasion. However, the carbides formed by        those elements are less effective than those formed by titanium        or zirconium and, for that reason, they are optional and added        in a limited quantity.    -   Optionally, one or more elements selected from selenium,        tellurium, calcium, bismuth and lead at contents of less than        0.1% each. Those elements are intended to improve machinability.        It should be noted that, when steel contains Se and/or Te, the        content of manganese must be such, taking into consideration the        content of sulphur, that manganese selenides or tellurides can        form.    -   The balance being iron and impurities resulting from the        production operation. The impurities include in particular        nitrogen whose content depends on the production method, but        generally does not exceed 0.03%. That element can react with        titanium or zirconium to form nitrides which must not be too        coarse in order not to inhibit the strength. In order to prevent        the formation of coarse nitrides, titanium and zirconium may be        added to the liquid steel in a very progressive manner, for        example, by placing in contact with the oxidized liquid steel an        oxidized phase, such as a slag charged with titanium or        zirconium oxides, then deoxidizing the liquid steel in order to        cause the titanium or zirconium to diffuse slowly from the        oxidized phase to the liquid steel.

Furthermore, in order to obtain satisfactory properties, the contents ofcarbon, titanium, zirconium and nitrogen must be such that:

0.1%≦C−Ti/4−Zr/8+7×N/8≦0.55%.

The expression C−Ti/4−Zr/8+7×N/8=C* represents the content of freecarbon after precipitation of the titanium and zirconium carbides,taking into consideration the formation of titanium and zirconiumnitrides. That free carbon content C* must be greater than 0.1%, andpreferably greater than or equal to 0.22%, in order to have martensitehaving a minimum hardness, but above 0.55% the strength and suitabilityfor use are excessively inhibited.

The chemical composition must further be selected so that thequenchability of the steel is sufficient, taking into consideration thethickness of the plate which it is desirable to produce. To that end,the chemical composition must comply with the following relationship:

Quench=1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>1.8 or moreadvantageously 2

with: K=0.5 if B>or equal to 0.0005% and K=0 if B<0.0005%.

It should be noted that, more particularly when Quench is from 1.8 to 2,it is preferable for the content of silicon to be greater than 0.5% inorder to promote the formation of retained austenite.

Furthermore, the contents of Ti, Zr and N must preferably be such that:Ti+Zr/2−7×N/2≧0.05% and more advantageously greater than 0.1%, and evenmore advantageously greater than 0.3%, so that the content of carbidesis sufficient.

Finally and in order to obtain good abrasion resistance, themicrographic structure of the steel is constituted by martensite orbainite or an admixture of those two structures and from 5% to 20% ofretained austenite, that structure further comprising coarse titanium orzirconium carbides, or niobium, tantalum or vanadium carbides, which areformed at high temperature. The inventors have established that theeffectiveness of coarse carbides for improving abrasion resistance couldbe inhibited by the premature separation thereof and that thatseparation could be prevented by the presence of metastable austenitewhich is transformed into new martensite under the effect of theabrasion phenomena. Since the transformation of the metastable austeniteinto new martensite is brought about by expansion, that transformationin the abraded sub-layer increases the resistance to separation of thecarbides and, in that manner, improves the abrasion resistance.

Furthermore, the great hardness of the steel and the presence ofembrittling titanium carbides make it necessary to limit insofar aspossible the planishing operations. From that point of view, theinventors established that, by slowing down the cooling sufficiently inthe region of bainitic/martensitic transformation, the residualdeformations of the products are reduced, which allows planishingoperations to be limited. The inventors established that, by coolingdown the workpiece or the plate at a cooling rate Vr<1150×ep^(−1.7) (inthis formula, ep is the thickness of the plate expressed in mm and thecooling rate is expressed in ° C./s), below a temperatureT=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2), (expressed in ° C.),firstly, the production of a significant proportion of residualaustenite was promoted and, secondly, the residual stresses broughtabout by the phase changes were reduced.

In order to produce a very planar plate which has good abrasionresistance, the steel is produced and cast in the form of a slab or bar.The slab or bar is hot-rolled in order to obtain a plate which issubjected to thermal processing which allows both the desired structureand good surface evenness to be obtained without further planishing orwith limited planishing. The thermal processing may be carried outdirectly in the rolling heat or carried out subsequently, optionallyafter cold-planishing or planishing at a medium temperature.

In order to carry out the thermal processing operation:

-   -   the steel is heated above the AC₃ point in order to confer on it        a completely austenitic structure,    -   then, it is cooled at a mean cooling rate greater than the        critical bainitic transformation rate as far as a temperature        which is equal to or slightly less than (by more than        approximately 50° C.) a temperature        T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2) (expressed in ° C.),    -   then, the plate is cooled, between the temperature defined in        this manner (that is to say, approximately from T to T−50° C.)        and approximately 100° C., at a mean core cooling rate Vr of        from 0.1° C./s, in order to obtain sufficient hardness, to        1150×ep^(−1.7), in order to obtain the desired structure,    -   and the plate is cooled as far as ambient temperature,        preferably but without being compulsory, at a slow rate.

Furthermore, it is possible to carry out a stress-relief processingoperation at a temperature less than or equal to 350° C., and preferablyless than or equal to 250° C.

In this manner, a plate is obtained whose thickness can be from 2 mm to150 mm and which has excellent surface evenness, characterized by adeflection which is less than 12 mm per metre without planishing or withmoderate planishing. The plate has a hardness of from 280 HB to 650 HB.That hardness depends principally on the content of free carbonC*=C−Ti/4−Zr/8+7×N/8.

In accordance with the contents of free carbon C*, it is possible todefine a plurality of ranges corresponding to levels of increasinghardness, and in particular:

-   a) 0.1%≦C*≦0.2%, the hardness is approximately from 280 HB to 450    HB,-   b) 0.2%≦C*≦0.3%, the hardness is approximately from 380 HB to 550    HB,-   c) 0.3%≦C*≦0.5%, the hardness is approximately from 450 HB to 650    HB.

Since the hardness is a function of the content of free carbon C*, thesame hardness can be obtained with very different contents of titaniumor zirconium. With equal hardness, the abrasion resistance becomeshigher as the content of titanium or zirconium becomes greater.Similarly, with an equal content of titanium or zirconium, the abrasionresistance improves as the hardness becomes greater. Furthermore, usingthe steel becomes easier as the content of free carbon decreases, butwith an equal content of free carbon, the ductility improves as thecontent of titanium decreases. All those considerations allow thecontents of carbon and titanium or zirconium to be selected that lead toall the properties which are most suitable for each field ofapplication.

According to the hardness levels, the uses are, for example:

-   -   from 280 to 450 HB: scoops, skips for lorries and dump trucks,        cyclone shielding, hoppers, moulds for aggregates,    -   from 380 to 550 HB: shielding for impact grinders, bulldozer        blades, grab bucket blades, grills for sieves,    -   from 450 to 650 HB: plates for shielding cylinder type grinders,        reinforcement elements for scoops, reinforcement elements under        leading blades, cut-water blade shields, leading edges.

By way of example, steel plates designated A to G according to theinvention and H to J according to the prior art are considered. Thechemical compositions of the steels, expressed in 10⁻³% by weight, aswell as the hardness, the content of residual austenite of the structureand a wear resistance value Rus are summarized in Table 1.

TABLE 1 C Si Al Mn Ni Cr Mo W Ti B N HB % aust Rus A 360 850 50 1300 500700 100 500 400 2 6 460 10 1.42 B 640 850 50 400 1500 700 110 450 620 37 555 14 2.72 C 590 520 570 550 320 1850 470 — 540 — 7 570 12 2.24 D 705460 630 1090 280 2450 430 100 825 — 7 580 13 3.14 E 690 370 25 740 3102100 460 — 795 — 6 605 10 2.83 F 350 810 30 1200 270 1350 380 160 2 6510 8 1.32 G 390 790 35 1210 250 1340 390 405 3 6 495 11 1.77 H 340 38030 1260 470 820 370 — 410 3 6 475 1 0.86 I 315 330 25 1230 180 1360 395165 2 6 515 2 0.7 J 367 315 30 1215 210 1375 405 430 2 5 500 2 1.01

The wear resistance value Rus varies as the inverse logarithm of theloss of weight of a prismatic test piece which is rotated in a containercontaining graded quartzite aggregate.

All the plates have a thickness of 30 mm and the plates corresponding tosteels A to G have been quenched in accordance with the invention, afteraustenitization at 900° C.

After austenitization, the cooling conditions are:

-   -   for the plates of steel B and D: cooling at a mean rate of 0.7°        C./s above temperature T defined above and at a mean rate of        0.13° C./s therebelow, in accordance with the invention;    -   for plates of steel A, C, E, F, G: cooling at a mean rate of 6°        C./s above temperature T defined above and at a mean rate of        1.4° C./s therebelow, in accordance with the invention;    -   for the plates of steel H, I, J, given by way of comparison:        austenitization at 900° C., followed by cooling at a mean rate        of 20° C./s above temperature T defined above, and at a mean        rate of 12° C./s therebelow.

The plates according to the invention have a martensitic/bainiticstructure which contains from 5% to 20% of retained austenite, whereasthe plates given by way of comparison have a completely martensiticstructure, that is to say, martensitic and not containing more than 2 or3% of retained austenite. All the plates contain carbides.

A comparison of the wear resistances shows that, with a similar hardnessand content of titanium, the plates according to the invention have acoefficient Rus which is on average 0.5 greater than that of the platesaccording to the prior art. In particular, comparison of examples A andH which substantially differ in terms of the structure (content ofresidual austenite of 10% for A, completely martensitic structure for H)shows the incidence of the presence of residual austenite in thestructure. It should be noted that the difference in content of residualaustenite results from both the difference between the thermalprocessing operations and the difference between the contents ofsilicon.

It can further be observed that, all things substantially being equalotherwise, the contribution to the wear resistance which can beattributed to the titanium carbides is significantly higher when theirpresence is combined with that of residual austenite in accordance withthe invention than when those carbides are precipitated within a matrixwhich is substantially free from residual austenite. Thus, for similardifferences in the contents of titanium (and therefore of TiC, thecarbon still being in excess), the pair of steels F,G (according to theinvention) differ distinctly from the pair of steels I,J in terms ofincrease in resistance brought about by the titanium. For F,G, theincrease in resistance Rus brought about by 0.245% of Ti is 0.46,whereas it is only 0.31 for a difference of 0.265% of Ti in the case ofthe pair I,J.

That observation can be attributed to the increased squeezing effect onthe titanium carbides by the surrounding matrix when it containsresidual austenite which can be transformed into hard martensite withexpansion under the effect of the abrasive stresses.

Furthermore, the deformation after cooling, without planishing, for thesteel plates according to the invention is less than 10 mm/m and isapproximately 15 mm/m for the steel plate H. In practice, that leadseither to the possibility of supplying the products without planishing,or carrying out planishing in order to comply with stricter requirementsin terms of surface evenness (for example, 5 mm/m), but which is carriedout more readily and with fewer stresses being introduced owing to thelesser original deformation of the products according to the invention.

1-9. (canceled)
 10. Workpiece, and in particular a plate, of steel whichis resistant to abrasion and whose chemical composition comprises, byweight: 0.35% ≤ C ≤ 0.8% 0% ≤ Si ≤ 2% 0% ≤ Al ≤ 2%0.35% ≤ Si + Al ≤ 2% 0% ≤ Mn ≤ 2.5% 0% ≤ Ni ≤ 5% 0% ≤ Cr ≤ 5%0% ≤ Mo ≤ 0.50% 0% ≤ W ≤ 1.00% 0.1% ≤ Mo + W/2 ≤ 0.50%0% ≤ B ≤ 0.02% 0% ≤ Ti ≤ 2% 0% ≤ Zr ≤ 4% 0.05% ≤ Ti + Zr/2 ≤ 2%0% ≤ S ≤ 0.005% N < 0.03% optionally from 0% to 1.5% of copper,optionally at least one element selected from Nb, Ta and V at contentssuch that Nb/2+Ta/4+V≦0.5%, balance being iron and impurities resultingfrom the production operation, the chemical composition furthercomplying with the following relationships:0.1%≦C−Ti/4−Zr/8+7×N/8≦0.55%andTi+Zr/2−7×N/2≧0.05%and:1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>1.8 with: K=0.5 ifB≧0.0005% and K=0 if B<0.0005%, whose surface evenness is characterizedby a deflection of less than 12 mm/m, the steel having a martensitic ormartensitic/bainitic structure, the structure further containing from 5%to 20% of retained austenite and carbides.
 11. Workpiece according toclaim 10, wherein:1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^(1/2)+K>2.
 12. Workpiece accordingto claim 10, wherein:C>0.45%.
 13. Workpiece according to claim 10, wherein:Si+Al>0.5%.
 14. Workpiece according to claim 10, wherein:Ti+Zr/2>0.10%.
 15. Workpiece according to claim 10, wherein:Ti+Zr/2>0.30%.
 16. Workpiece according to claim 10, wherein:C*≧0.222%with C*=C−Ti/4−Zr/8+7×N/8
 17. Workpiece according to claim 10, whereinit is a plate having a thickness of from 2 mm to 150 mm and whosesurface evenness is characterized by a deflection of less than 12 mm/m.18. Workpiece according claim 10, wherein the hardness is from 280 HB to450 HB and:0.1%≦C−Ti/4−Zr/8+7×N/8≦0.2%.
 19. Workpiece according to claim 10,wherein the hardness is from 380 HB to 550 HB and:0.2%<C−Ti/4−Zr/8+7×N/8≦0.3%.
 20. Workpiece according to claim 10,wherein the hardness is from 450 HB to 650 HB and:0.3%<C−Ti/4−Zr/8+7×N/8≦0.5%.